DiffPDF is the open source software by which you can compare two PDF files for the difference.This is similar tools like WinMerge and DiffMerge that compare two folders & compare text files for the difference respectively.DiffPDF allows user to compare the PDF files either based on appearance or based on text accurately.Here is the screenshot of an image showing comparison between two PDF files.

As I told earlier,You can compare your PDF Documents either by textually or visually.Text mode can be used to compare the files having only text,it will not useful to compare the documents having images in it.And By Appearance mode allows user to compare the PDF file having images in it.